More energy from the sun strikes the Earth in one hour than all the energy consumed on the planet in one year, yet solar electricity accounts for less than 0.02% of all electricity produced worldwide. The enormous gap between the potential of solar energy and its use is due, in part, to the cost/conversion capacity. The development of third generation solar cells (high efficiency plus low cost) is of paramount importance to both humanity and nature.
Solution-processability has been recognized as a feasible solution to cost issues, and novel mechanisms such as carrier multiplication a possible route to achieve higher efficiency levels. In both these aspects, there is potential in colloidal infrared quantum dots, such as lead selenide (PbSe) and lead sulfide (PbS).
Quantum dots (QDs) are nanocrystals, consisting of tens to hundreds of atoms, whose size is smaller than its bulk exciton Bohr radius (46 nm for PbSe and 18 nm for PbS), as seen in FIG. 1(B). Due to the nanocrystal's small size (smaller than the exciton Bohr radius of the bulk semiconductor), strong quantum confinement results in discrete energy levels and a larger bandgap than the respective bulk semiconductor, as seen in FIG. 2. The first excitonic transition (E1) from 1Sh to 1Se defines the optical band gap Eg of a nanocrystal. For PbSe QDs, depending on the size, their bandgap energies can be tuned from 0.5 eV to 1.2 eV, as seen in FIG. 3 (Murray & Bawendi, Synthesis and characterization of nearly monodisperse CdE (E=sulfur, selenium, tellurium)semiconductor nanocrystallites. J. Am. Chem. Soc., 1993. 115: p. 8706; Murray, et al., Colloidal synthesis of nanocrystals and nanocrystal superlattices. Ibm Journal of Research and Development, 2001. 45(1): p. 47-56; Pietryga, et al., Pushing the band gap envelope: Mid-infrared emitting colloidal PbSe quantum dots. Journal of the American Chemical Society, 2004. 126(38): p. 11752-11753; Yu, et al., Preparation and characterization of monodisperse PbSe semiconductor nanocrystals in a noncoordinating solvent. Chem. Mater, 2004. 16: p. 3318; Murray, et al., “Synthesis and Characterization of Monodisperse Nanocrystals and Close-packed Nanocrystal Assemblies”, Annu. Rev. Mater. Sci. 2000, 30, 545-610). Infrared QDs such as PbSe and PbS synthesized via a solution-based procedure provide an alternative platform for low cost photonic devices. Near infrared colloidal QD photodetector offers higher sensitivity than its bulk materials due to the narrow D* spectrum (Konstantatos, et. al., Ultrasensitive solution-cast quantum dot photodetectors, Nature. 2006, 442, 180-183). The electroluminescent powers of the best PbS QD LEDs were comparable to commercial products (Sun, et al., Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control, Nat. Nanotechol. 2012, 7, 369-373). PbSe and PbS QDs have generated particularly strong research interest in photovoltaic applications, due in part to the discovery of multiple exciton generation (MEG) that possibly could boost QD solar cell efficiency (Schaller & Klimov, High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion, Phys. Rev. Lett. 2004, 92, 186601-4; Ellingson, et al., Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots, Nano letters. 2005, 5, 865-871; Beard, Multiple Exciton Generation in Semiconductor Quantum Dots, J. Phys. Chem. Lett. 2011, 2, 1282-1288). MEG based solar cells were fabricated and the effect confirmed in recent reports (Semonin, et al., Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell, Science. 2011, 334, 1530). Despite its poor initial power conversion value of <0.01% in efficiency in QD hybrid solar cell with polymers in 2004 (McDonald, et al., Solution processed PbS quantum dot infrared photodetectors and photovoltaics, Nature Materials. 2004, 4, 138-142), followed by years of faltered faith in this class of materials due to the frustration with MEG phenomenon (Nair, et al., Carrier multiplication yields in PbS and PbSe nanocrystals measured by transient photoluminescence, Physical Review B. 2008, 78 (12), 125325; McGuire, et al., New Aspects of Carrier Multiplication in Semiconductor Nanocrystals, Accounts of Chemical Research. 2008, 41 (12), 1810-1819; Pijpers, et al., Assessment of carrier-multiplication efficiency in bulk PbSe and PbS, Nature Physics. 2009, 5, 811) and declined research interest partially because of the disappointing results on QD hybrid solar cells (Maria, et al., Solution-processed infrared photovoltaic devices with >10% monochromatic internal quantum efficiency, Appl. Phys. Lett. 2005, 87, 213112-213112; Watt, et al., Lead sulfide nanocrystal: conducting polymer solar cells, J. Phys. D. 2005, 38, 2006-2012; Jiang, et al., Nanocomposite Solar Cells Based on Conjugated Polymer/PbSe Quantum Dot, Proc. Of SPIE. 2005, 5938, 59381F-1; Gunesac, et al., Hybrid solar cells using PbS nanoparticles, Solar Energy Materials & Solar Cells. 2007, 91, 420-423), in 2012 a record of 7.4% power conversion efficiency PbS QD solar cell was reported (Ip et. al., Hybrid passivated colloidal quantum dot solids, Nature Nanotechnol. 2012, 7, 577-582), a value that is close to commercially viable products.
Despite being one of the most promising solutions for solar energy utilization, the present performance of such infrared quantum dot-based PV devices are far from their expectations. The two major causes for their relatively low efficiency have been recognized in current research; inefficient exciton separation at the quantum dot/constituent interface and poor charge percolation pathways to the extracting electrodes. Efficient photo-induced charge transfer has not been observed in quantum dot composites, due largely to the lack of a measurement technique which would allow a clear separation between exciton dissociation and charge transport phenomena. This makes it challenging to gain detailed insight into either phenomenon, impeding rational design of absorber layers. Furthermore, the majority of the transport studies so far have been limited to the planar structure field effect transistors (FET), whereas an applicable quantum dot photovoltaic (PV) device is of sandwich structure, and it is known that the transport characteristics could be very different in these two structures.
The common breakthrough in all three types of photonic devices was ligand manipulation. During the colloidal synthesis process, certain ligands (usually TOPO or oleic acid) are used to passivate quantum dot surfaces to prevent aggregations. The as-synthesized PbSe QDs are practically insulators due to the original bulky passivation ligands, such a oleic acids depicted in FIG. 1(A), which are used to separate QDs. These ligands are barriers in electronic processes such as exciton dissociation, charge transfer and transport. Incomplete passivation results in surface trap sites, with the bulky ligands serving as barriers for exciton dissociation at the quantum dot/constituent interface, thereby hindering photo-induced charge transfer (PCT). The addition of quantum dots without the optimization of their interfaces with other constituent(s)—i.e., without the formation of separate percolation pathways for electrons-e and holes-h—causes huge loss of the photo-generated free carriers due to e-h recombination. Thus, to break through the present technology limitations, these ligand barriers must be reduced while making QDs electronically coupled.
Conventionally, post-synthesis treatments, including chemical and thermal treatments, have proven to be efficient to enhance quantum dot transport properties. Usual chemical treatments include ligand exchange in solution prior to film formation and ligand condensation or removal on films. Remarkable increases in film conductivity and mobility have been achieved by hydrazine treatment and EDT (1,2-ethanedithiol) soaking (Talapin & Murray, PbSe Nanocrystal Solids for n- and p-Channel Thin Film Field-Effect Transistors, Science. 2005, 310, 86-89; Luther, et. al., Structural, Optical, and Electrical Properties of Self-Assembled Films of PbSe Nanocrystals Treated with 1,2-Ethanedithiol, ACS Nano. 2008, 2, 271-280).
Improved infrared response in PbS quantum dot photovoltaic devices and photoconductors has been demonstrated after ligand exchange with butylamine (Johnston, et. al., Schottky-quantum dot photovoltaics for efficient infrared power conversion, Appl. Phys. Lett. 2008, 92, 151115). Moreover, photodetection at SWIR wavelength of 1.3 μm with a normalized detectivity (D*)>s1013 jones has been demonstrated with butylamine-exchanged PbS NCs (Konstantatos, et. al., Ultrasensitive solution-cast quantum dot photodetectors, Nature. 2006, 442, 180-183).
However, there are serious limitations related to conventional treatments. Firstly, the high D* (Konstantatos, et. al., Ultrasensitive solution-cast quantum dot photodetectors, Nature. 2006, 442, 180-183) had to be compromised by a slow response (hundreds of milliseconds) due mainly to the deep trap states brought in by chemical treatments. Secondly, ligand exchange is very hard to control, and usually it ends up with partial exchange and loss of free volume leading to development of cracks in the nanocrystal films (Konstantatos, et. al., Ultrasensitive solution-cast quantum dot photodetectors, Nature. 2006, 442, 180-183; Seo, et. al., Enhancement of the photovoltaic performance in PbS nanocrystal:P3HT hybrid composite devices by post-treatment-driven ligand exchange, Nanotechnol. 2009 20, 095202; Hanrath, et. al., PbSe Nanocrystal Network Formation during Pyridine Ligand Displacement, ACS Applied Materials & Interfaces. 2009, 1, 244). Thirdly, ligand removal processes involve very harmful chemicals such as hydrazine, and are not practical with solution processes since the ligands removal is performed on QD film. Thermal treatments have shown to improve carrier mobility and conductance (Romero & Drndic, Coulomb Blockade and Hopping Conduction in PbSe Quantum Dots, Phys. Rev. Lett. 2005, 95, 156801; Law, et. al., Structural, Optical, and Electrical Properties of PbSe Nanocrystal Solids Treated Thermally or with Simple Amines. J. Am. Chem. Soc. 2008, 130, 5974-5985; Mentzel, et. al., Charge transport in PbSe nanocrystal arrays, Phys. Rev. B 2008, 77, 075316; Urban, et. al., Self-Assembly of PbTe Quantum Dots into Nanocrystal Superlattices and Glassy Films. J. Am. Chem. Soc. 2006, 128, 3248-3255), however, temperature elevation was shown to cause desorption of ligands, and the presence of large organic ligands after thermal annealing may increase the density of trap sites and prevent the ordered arrangement of QDs (Baik et. al., Low-Temperature Annealing for Highly Conductive Lead Chalcogenide Quantum Dot Solids, J. Phys. Chem. C. 2011, 115, 607-612; van Huis, et. al., Low-Temperature Nanocrystal Unification through Rotations and Relaxations Probed by in Situ Transmission Electron Microscopy. Nano Lett. 2008, 8(11), 3959-3963), which would cause a decrease in carrier lifetime and mobility (Tisdale, et. al., Hot-Electron Transfer from. Semiconductor Nanocrystals. Science. 2010, 328, 1543-1547; Pandey & Guyot-Sionnest, Slow Electron Cooling in Colloidal Quantum Dots. Science. 2008, 322, 930-932). Furthermore, there was evidence showing that thermal treatment caused the QDs to lose their crystalline features (Law, et. al., Structural, Optical, and Electrical Properties of PbSe Nanocrystal Solids Treated Thermally or with Simple Amines. J. Am. Chem. Soc. 2008, 130, 5974-5985; Mentzel, et. al., Charge transport in PbSe nanocrystal arrays, Phys. Rev. B 2008, 77, 075316; Urban, et. al., Self-Assembly of PbTe Quantum Dots into Nanocrystal Superlattices and Glassy Films. J. Am. Chem. Soc. 2006, 128, 3248-3255).
Recently, promising results on QD solar cells have been achieved by using short ligands during the synthesis process, including use of atomic ligand passivation (Tang, et. al., Colloidal-quantum-dot photovoltaics using atomic-ligand passivation. Nature Mater. 2011 10, 765-771), and the use of hybrid passivation ligands (Talapin & Murray, PbSe Nanocrystal Solids for n-and p-Channel Thin Film Field-Effect Transistors, Science. 2005, 310, 86-89). Power conversion efficiency in the range of 3.5-4.6% has been reported with very small PbSe QD (<2 nm) solar cells (Ma, et. al., Photovoltaic Performance of Ultrasmall PbSe Quantum Dots. ACS Nano, 5, 8140-8147, 2011). Yet, comparing with the original simple synthesis route using oleic acid or TOPO as passivation layers (Yu, et al., Preparation and characterization of monodisperse PbSe semiconductor nanocrystals in a noncoordinating solvent. Chem. Mater, 2004. 16: p. 3318; Murray, et al., “Synthesis and Characterization of Monodisperse Nanocrystals and Close-packed Nanocrystal Assemblies”, Annu. Rev. Mater. Sci. 2000, 30, 545-610), these new synthesis processes, such as hybrid passivation synthesis of QDs, are more complex and harder to control. (Ip, et al., Hybrid passivated colloidal quantum dot solids. Nat Nanotechnol, 2012; 7. 577-582), due to the unspecified ‘hard to access sites’ in solution phase, and the complexity of using ‘bidentate organic linkers’ during film formation phase.
The addition of surface passivized quantum dots to present cost-effective organic solar materials (i.e., polymers) could double power conversion efficiency to twelve percent due to the infrared absorbers' enhanced spectrum match with sunlight. Initial observation of carrier multiplication and recent confirmation of it in these quantum dots holds fundamental importance in current solar cell development. Ligands are hindrance of QD devices. It is necessary to have surface passivation during colloidal synthesis process. However, an innovative method is called to for a complete elimination of ligands in QD devices.